FIG. 1A illustrates a fundamental circuit diagram of a passive pixel commonly used in digital radiographic detectors for x-ray imaging. An exemplary portion of a two dimensional passive pixel array is illustrated in FIG. 1B, wherein a 2×3 array is made up of pixels as shown in FIG. 1A.
The pixel of FIG. 1A comprises a photo-sensing element (photodiode) and a readout element (TFT). In the array schematic shown in FIG. 1B, the photo-sensing element is a PIN photodiode and the switching device is a TFT with source, gate and drain, although other photo-sensing elements and alternative switching elements may be employed. In the circuit of FIGS. 1A-1B the anode of the diode is connected to a bias supply VBIAS. The cathode of the diode is connected to the drain of the TFT. The gate of the TFT is controlled by a gate line. The gate line is oriented along the (horizontal) row direction and typically connects the gates of all the pixels in a row to a row address circuit, or row select circuit. The row address circuit, which is positioned peripheral to the array, sequentially addresses each row, momentarily switching the TFT in the pixels along that row from an insulating (off) into a conducting (on) state. The source of the TFT is connected to a data line, which is oriented in the (vertical) column direction of the array and is typically connected to all pixels in that column. Each data line is connected to a signal sensing circuit peripheral to the array. In the circuit of FIG. 1B, the signal sensing circuit comprises a charge amplifier including an operational amplifier (op amp), a feedback capacitor (Cf), a reference voltage supply (Vref), and a reset switch (RS). The charge amplifier senses the amount of charge required to reset the data line to the reference voltage by measuring the charge on the feedback capacitor.
In typical operation, the cathodes of all photodiodes in the array are reset to VREF by sequentially addressing each row of pixels. The photodiode voltage is thus reset to VBIAS−VREF. In the presence of X-ray exposure, the photo-charge is stored on the photodiode. Following exposure the charge in the array may be read out by sequentially addressing each of the rows, transferring the charge in the photodiodes in that row to the respective data lines, and sensing the charge in the charge amplifier connected to each of the data lines.
For successful operation, it is important that the TFT have high resistance between source and drain in the “off” state to prevent charge leakage from the photodiode to the data line. The leakage current IDS through the TFT from the photodiode to the data line in each pixel in the “off” state is given by IDS=Roff(VGoff)·VDS=Roff(VGoff)·Qphoto/CPD where Roff is the off-state resistance of the TFT at the gate voltage VGoff and VDS is the voltage difference between source and drain produced by the photo-charge Qphoto and the photodiode capacitance CPD. In typical digital radiographic imaging arrays a value of Roff(VGoff)>1014Ω is desired to prevent loss of signal charge prior to array readout.
It is also important that the TFT have low resistance between drain and source in the “on” state Ron(VGon) in order to minimize the time required to transfer charge from the photodiode to the data line. The time constant τRC for the transfer of charge is given by τRC=RTFT(VGon)CPD where RTFT(VGon) is the resistance between TFT source and drain (typically called the channel resistance) at a gate voltage of VGon and CPD is the photodiode capacitance. Switching the TFT gate to VGon for three or more time constants τRC allows 95% or more of the stored charge to be transferred from the diode to the data line.
For typical radiographic imaging arrays, the photodiode capacitance CPD is in the range of 1-3 pF and the TFT materials and dimensions are chosen to achieve RTFT of about 0.5-5 MΩ in order to achieve τRC in the range of 1-10 μs. This allows imaging arrays with greater than 3,000 rows to be read out in a time significantly less than 1 second.
Amorphous silicon thin-film transistors (a-Si TFTs) have been widely used in digital radiographic detectors. A-Si TFTs typically have excellent uniformity both over an individual array and from array to array. They also are stable over the total-dose of radiation exposure over the life of a radiographic detector. However, they have low electron mobility, which leads to a long charge transfer time constant τRC. Although the τRC may be reduced by increasing the width of the a-Si TFT, this results in increased TFT gate-to-source capacitance CGS, which causes high data line capacitance and thereby high noise and high clock feedthrough. Alternative semiconductor materials for TFT's include metal oxide TFT's, such as InGaZnO4 or low-temperature polysilicon (LTPS), which display significantly higher mobility than a-Si TFT's. TFT's fabricated with InGaZnO4 have mobility about 20× higher than a-Si, allowing faster and lower noise radiographic detector arrays. FIG. 2A shows a cross-section of a typical bottom-gate InGaZnO4 transistor and FIG. 2B shows the transfer characteristics of an InGaZnO4 transistor with gate length of 5 μm and gate width of 20 μm. The graph on the left illustrates the log of drain current (Id in amps) vs. gate voltage (Vg in V) with different levels of drain-source voltage (Vds) at 0.1V, 1V, 5V, and 10V; the graph on the right illustrates the drain current in mA vs. drain voltage (Vd in V) with different levels of gate-source voltage (Vgs) at 5V, 10V, 15V, and 20V. The threshold voltage (VT) is the voltage at which the transistor switches between low and high resistance states. The TFT of FIGS. 2A-2B may be switched to the “off” state by applying a gate voltage VGoff of about −5V to −10V and may be switched to the “on” state by applying a gate voltage VGon of +10V to +30V. However, InGaZnO4 TFT's display significantly higher threshold voltage variability both spatially within an array as well as variability from array to array. InGaZnO4 TFT's display instability in threshold voltage with electrical stress. They also display negative VT shift with radiation exposure. TFT's fabricated with LTPS have mobility about 100× higher than a-Si. However, LTPS TFT's have very large variability in threshold voltage from one TFT to another TFT even on a short spatial scale. They also display variability in threshold voltage across an array, and from array to array. The threshold voltage is also very sensitive to temperature and displays instability with electrical stress and radiation exposure.
Prior-art arrays may utilize a single value of VGon and a single value for VGoff which is typically set during manufacturing and expected to remain unchanged for the life of the product. For successful operation of the imaging array using a single value of VGoff and a single value of VGon for all TFT's in the array, the TFT's must have values of Ron and Roff that are uniform over the array, stable over the operating environment of the array, and stable over time. If the threshold voltage of the TFT is highly non-uniform over the array, if it changes with operating environment parameters such as temperature, or if it changes over the life of the product by factors such as radiation damage, some TFT's may display reduced Roff, causing photodiodes to leak charge onto the dataline even when they are not addressed, while others may display low Ron, causing incomplete transfer of charge from the photodiode to the data line and thereby causing image lag and ghost images in subsequent frames.
The problem of non-uniform threshold voltage and the problem of unstable threshold voltage is especially observed in high performance TFT devices, such as devices fabricated from re-crystallized polysilicon (LTPS) or metal-oxides (such as InGaZnO4). These high performance TFT devices allow faster readout, smaller size, lower noise, and lower operating voltages than TFT's produced with a-Si. However, the high performance TFT devices are typically less uniform spatially both locally and globally within an array, less uniform from array to array and from glass substrate to glass substrate. High performance TFT devices are also typically more sensitive to radiation exposure. For successful operation of arrays it can be seen that circuits and methods of operation for calibration and correction of threshold voltage would be beneficial.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.